Method of depositing silica films and preparation of solutions therefor



Mpm H, R950 s. M. THOMSEN ETAL 2,505,629

METHOD 0F' DEPOSITING SILICA FILMS AND PREPARATION SOLUTIONS THEREFOR Filed June 50, 1949 Patented pr. 25,

METHODF DEB OSITINGSILICAFILMS AND` .QE SOLUTIQN THEREFOR Soren M. Thomsen, Pennington, and ErederickH.- Niooll, 'PrincetonyNiM assgnors to Radio Cor--` poration` of.Amei'.ica,- a corporation of De`laware.

Applicatinguneszo,194e; serial 1510.102262 1p claires., (o1. 11.7.-124).

Thisjnvention relates. to. improvements in.V the.:`

method in which quartz .is heatedtoa. temperature high enough to vaporiae.thesilica.` Because. ofthe extremely high.temperatureatlwhoh silica. vaporizes, however, this is..acostlyanrl.diioulti methodto use. It has also. been proposed-.ladder posit silica fromA organic. silicates, alkali silicates..

and from the vapor oi decomposablev silicon..com, pounds such as silicontetrachloride. Although, these methods have beenmoreor. lesssuccessful for some commercial applications, it.. has .pre1/en..

to be Very diicultto deposit an.evenciilIriroffsilicsi,V -s

having an accurately controlled1 thiclness .byganye ofthese older methods.

One object of the present.inventionistq provide,- an improved method of depositing verythinfllms. of silica havingV accurately controlled thiclmesses Another object. is to .provide a method oi cie-,-i positing transparent .ilms.. of. silica with the Ve::.,- penditure of very little` mechanical energy Another object is to. provide an impllwd.; method of protecting. glass, surfaces` with thin, silica lms.

Another object is` to. provide an ilnproyetl method of depositing .silica-from. a. solution...`

Still anotherlobiect is toprovide. an. illil1-oved;Y mcthodlof depositing, reilection-reducingdilms o isilica on transparent optical surfaces..y

lheseand other objects will become rxiorebap;l parent and the invention will .be bettenunderg, stoodfrom a studyA of the. descriptionoilthe .in vention which follows togetherwiththegaccorn;1 panying drawings, of which,

Fig. 1V is a graph showing the ratego ieposift tion of a silica llm deposited `from atypical .t ate;V ing.' bath made up according tothe presentinvenf; tion.

Fig. Zis a graph showing the time necessarytoV deposit-.a 1A wavelength lm from iluosilicic acid. solutions .adjusted to various degrees of.super saturation with respect to silica.

. 2; solubility with concentration OLHZSiFa taking af 1.25! molar solution of. HzSiFs as a zerofreferl-I ence point. Y

It has been toundra-that a solution oitluosilicicl acidsupersaturated with just the correctamountf of silica Willdeposit a verythin, uniform, hard, transparent film of silica upon anobject-immersed? inthe solution. This isinfcontrastlto a uo'silicici acidL solution which is'- eitherl just saturated? or un'dersaturatedlwith fsilica since these latter types-- slowly attack most `glasses instead, of depositingi4 silica on them and, of course, donotdeposit-sili'ca` on other surf aces immersed in them. The method-.1 of the present invention is also incontrast. to the? methbd-` of skeletoniz'ingl the surface of a glass article in order'to produce a reilectionreducina nlm which is-` described in co-pending` applica. tionfSerial No. 698,441, ledlSeptember 21, 194:6;` now Patent No. 2,490,662?. The skeletonizing proc-f ess also utilizes a solution of fluosilicic acid-supersaturated with respect to silicabut the range. of supersaturation is somewhat diflerentlin the'twol cases. The close similarityi between the two types of solution, howeverfsrves'to indicate the ne degrecofcontrollnecessary to obtain lmsunde the present invention 4andi-the unexpectedness ofsthe results obtainedin the process.

ofcourse, one -general-methodof depositingia substance chemically is to supersaturate a soliti# tion with respect to the substance-then iritrc'idue`v conditions which will causethe excess substance:y to" precipitate out of-the solution. This generali method can also be used to obtain a depositl of` silica as is lwellknownpf However, as isfedually well known; the deposit of-i-silic'ai-wh-ich i5 usually" formed is a gelatinous" mass which is o'f-aLcloudyf ortanslucent nature. By practicing thevmethodl of L the" present invention', an optically thin lm may" be formed which is` hardand` d` t'ransparer'it.` This hlm-may be" so accurately controlledvastobe of -the order of lpwavelength in tlfiick-nessof;A

some desiredicomponent of white. light. onit.may;` be much thicker.in1ordertonbetter serve .as aprile... tectii'fezcoating.V Ifit isto. be. used as a..reiiectiom` reducing. llm, it may,.as.is.wel1lknown, bepnjlade..` a. small, odd multiple` of 1iswavelength inlthicl-y ness.

TheA method of the` present invention ,ieperformed by immersing the article `to be coatedinra,` solution of fluosilicic acid which has ibeenmale,` supersaturatedwith respect tosilica within the range ofaboutA to about 16 millimoles per literL; A preferred range of excess silica is about 8to` about lmillimolesper liter and a preferredpoint.

Fis.. Brisa graph .0i-the. chasse in. .erassf-sl1lsai5t vitiliehisl fange iS-.ebQei10-1111im0leS sie??? saturation per liter. The iiuosilicic acid appears to act as a catalyst in the deposition of the silica.

The method of making up the treating solutions can be accomplished in several ways. It must first be pointed out that, although nuosilicic acid is generally described in the literature as conforming to the ideal formula HzSiF, it has been unexpectedly found that commercial fluosilicic acid is capable of dissolving considerable excess silica so that the final stable solution obtained has a ratio of iiuorine to silica of nearer 5 to 1 on a gram equivalent basis than the 6 to l indicated by the theoretical formula. This saturated solution is obtained by taking commer-.

cial iiuosilicic acid and dissolving in it all the silica it will take at 25 C. Actually, the fluosilicic acid is allowed to remain in contact with Solid reagent silicic acid for an hour or two inv order to put in the added amount of silica. This saturated solution is then made supersaturated to the required degree. In order to do this, it may simply be diluted with water, or borc acid may be added to reduce the solubility of the silica or sodium silicate may be added to introduce the excess silica.

. The relative amount of silica, over and above that required by the formula HzSiFe, which is present in saturated solutions of various concentrations of uosilicic acid in moles per liter is illustrated in Fig. 3. of experimental results obtained using a solution containing 1.25 moles per liter of HzSiFG as a zero reference point. The graph shows that as the concentration of the iiuosilicic acid in moles per liter rises, increasing amounts of silica are 3 dissolved per mole of HzSiFs. For example, the graph shows that a 2.5 molar solution of HasiFs will dissolve almost 12 millimoles more silica per mole of l-lzSiFe than will a 1.25 molar solution of the acid.

Specific examples of making up the treating solution follow:

Example 1 A liter of 2.5 molar solution of uosilicic acid is saturated with silicio acid at 25 C. This is then'diluted with an equal volume of water to makey up two liters of 1.25 M solution. The resulting solution has now become supersaturated With silica to the extent of about 12 millimoles per mole of HzSiFs or about l5 millimoles per literof solution and deposits silica smoothly and uniformly.

f This is so since the liter of 2.5 molar solution when diluted to form 2 liters of 1.25 molar solution still has the higher amount of silica which the 2.5 molar saturated solution was able to dissolve. The curve of Fig. 1 illustrates the rate of deposition of silica on a glass surface brought about by immersing a glass plate in this solution. .As shown in the igure, a film which is 1/4 wavelength of green light of 5200 in thickness deposits in about l4 hours while one which is 1/2 wavelength thick deposits in about 30 hours. The rate of deposition decreases somewhat as the time increases. Thickness of deposit was determined visually from the interference color.

In order to compare the results of diluting the 2.5 molar solution of fiuosilicic acid saturated with silica, with varying amounts of water, samples of the base solution of Example 1 were diluted with water to give solutions containing from to 90 percent of the 2.5 M acid. Most of these dilutions did not give satisfactory lms comparable to those obtained by diluting to about This figure is a graph fr.'

50 percent 2.5 M acid strength since either too much or too little excess silica was caused to appear in the solution. Those, with substantially greater amounts of water, being too supersaturated with respect to silica (or silicio acid) simply produced a heavy white precipitate while those with substantially less water took unreasonably long periods of time although they deposited silica very slowly.

Example 2 To a 1.4 M uosilicic acid solution, which is saturated with silica at 25 C., various amounts of boric acid were added in quantities ranging from 20-40 cc. of 4 percent boric acid per liter of uosilicic acid solution. The time required for amounts within this range to deposit a film having a thickness of 1/4 wavelength of green iight of 5200 at 55u C. is shown in Fig. 2. The addition of 2O cc. of the 4 percent boric acid per liter of the uosilicic acid solution results in a solution supersaturated with respect to silica to the amount of 8 millimoles per liter. The 40 cc. addition caused a super-saturation of i6 millimoles. When quantities of boric acid were used sufficient to 'provide more than about 16 millimoles excess silica per liter a cloudy white deposit of silica formed instead of a hard, transparent nlm. Amounts substantially lower than 20 cc. resulted in film formation only over long periods of time and no film formation at all, in many instances, when the amount used was such as to reduce the silica supersaturation to less than about 4 millixnoles per liter.

The examples given merely illustrate two ways in which a iluosilicic acid solution saturated with silica may be made supersaturated. Obviously, other equivalent reagents may be used. The concentration of the fluosilicic acid solution is not at all critical within ordinary limits. Obviously, a solution which is unreasonably dilute is not practical because of the diiiiculty or even impossibility of adjusting it to the correct amount of supersaturation with respect to silica While concentration above 2.5 molar cannot be obtained commercially at present.

Rate of deposition of silica also depends upon the temperature of the solution, the rate being more rapid at higher temperatures. When the temperature is too high, the deposit becomes milky. The amount of silica needed to supersaturate the iiuosilicic acid solution to the required extent, of course, changes also with temperature. However, it has been found that the change is quite irregular and is diiferent for different molar concentrations of fluosilicic acid. It has been found, for example, that in the case of a 1.25molar solution there is no difference in the required amount of silica between a solution at 25 C. and one at 45 C. But at other concentrations, the amount does change and for most concentrations above 1.25 molar the amount is actually'less than at 25 C. But, regardless of specic concentration of the uosilicic acid within the workable limits or of ordinary temperatures of deposition, the range of supersaturation withl respect to silica (or silicic acid) remains the same for the formation of good deposits Within reasonable periods of time. Optimum range of temperatures to observe for good deposition has been found to be about 25 C. to about 55 C. Temperatures at least as high as C. may be used at relatively low concentrations of iiuosilicic acid and lower temperatures than 25 C. may be used especially when relatively high concentrations of fluosilicic acid are present. It is not practical to use the higher ranges of concentration when also using higher temperatures, however, since solutions are likely to become turbid and unt for good deposition.

Films produced by deposition of silica by the method described are hard and smooth. Their index of refraction is 1.46 corresponding to solid silica. They appear to deposit Well on any kind of glass or other ceramic surface. On glass having an index of refraction of 1.52, a 1/4 Waveiength nim (with respect to green iight of 5200 has a blue interference color and has a rellectivity only 59 percent of that of the clean glass with no film. While this is not as great a reduction in reflectivity as that produced by some other methods due to the relatively high index of refraction of the lm, the durability is excellent and the lm is relatively inexpensive to apply.

The films can be deposited on surfaces other than those ci a ceramic nature. For example, they will form on plastics. Among those giving especially good results are the urea formaldehyde and the phenolic plastics. In general, it may be said that the lms can be deposited with varying degrees of excellence upon any substance not attacked by fluosilicic acid. They are useful for putting a protective surface on the article which renders it comparatively immune to attacks by many corrosive liquids and atmospheric influences.

There has thus been described an improved method of depositing hard, transparent, silica films from a controlled solution. The lms may be made so thin that they will reduce reflection from a glass surface or they may be made thick enough to serve as protective coatings on various surfaces.

We claim as our invention:

l. A process of depositing a. clear, hard, uniform lm of silica on a glass surface, comprising immersing a glass article to be coated in a solution of fluosilicic acid supersaturated with respect to silica to the extent of about e to about 16 millimoles of silica per liter of solution and allowing the article to remain in said solution until a coating of desired thickness has formed.

2. The process of claim 1 in which said excess of silica is about millirnoles Der liter.

3. The process of claim 1 in which said silica lm is a reflection-reducing coating having a thickness which is a small odd multiple of A wavelength of light of predetermined wavelength.

4. A method of preparing a solution for the deposition of clear, hard films of silica, comprising saturating a luosilicic acid solution with silica and then supersaturating said solution with silica to the extent of about 4 to 16 millimoles of silica per liter.

5. The method of claim 4 in which said supersaturation is accomplished by diluting the saturated solution with water.

6. The method oi claim fi in which said supersaturation is accomplished by adding to said saturated solution a soluble silicate.

'1. A process of depositing a lm of silica on a ceramic surface comprising immersing an article having said ceramic surface in a solution 0f uosilicio acid having a concentration of about 1.25 molar and which is supersaturated with respect to silica to the extent of about 4 to about 16 millimoles of silica per liter of solution and allowing the article to remain in said solution until a coating of desired thickness has formed.

8. The process of claim 7 in which said ceramic surface is glass.

9. A method 0f preparing a solution for the deposition of clear, hard ilms of silica comprising saturating a solution of iluosilicic acid with silica and then supersaturating said solution with silica by adding a small amount oi boric acid.

10. A process oi depositing a, transparent, hard, lrn of silica on a glass surface, comprising immersing a glass article t0 be coated in a solution of fluosilicic acid having a molar concentration of between about 1.25 and about 4 and which is at a temperature not exceeding about 76 C., said solution being supersaturated with respect to silica to the extent oi about 8 to about 16 millimoles of silica per liter of solution, and allowing said article to remain in said solution until a. coating of desired thickness has formed.

SOREN M. THOMSEN. FREDERICK H. NlCOLL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Geffcken Jan. 2, 1945 OTHER REFERENCES Number 

10. A PROCESS OF DEPOSITING A TRANSPARENT, HARD, FILM OF SILICA ON A GLASS SURFACE, COMPRISING IMMERSING A GLASS ARTICLE TO BE COATED IN A SOLUTION OF FLUOSILICIC ACID HAVING A MOLAR CONCENTRATION OF BETWEEN ABOUT 1.25 AND ABOUT 4 AND WHICH IS AT A TEMPERATURE NOT EXCEEDING ABOUT 70*C., SAID 